A number of attempts have been made over the years to develop a process that is capable of effectively and cost efficiently oxidizing a variety of feed materials. Many of these processes were initially developed for use in smelting or the removal of metal from ores. These processes consumed large amounts of energy, emitted noxious gases, and rarely achieved complete recovery of all the metals entering the process. They were also limited to very specific uses related to smelting, which made them largely unsuitable for use with other feed materials.
Other processes have also been developed to oxidize various feed materials. One in particular was an aqueous phase oxidation process that oxidized a feed material in a solution of nitric and sulfuric acid. The reaction occurred in a pressurized reactor that was maintained at a temperature no greater than about 210° C. Oxygen gas was added to reoxidize a substantial portion of the reduction products of nitric acid that were formed during oxidation of the feed materials.
Although this process was a significant advance over conventional techniques at the time, it still suffered from a number of problems. For one, the process used a significant amount of oxygen gas to oxidize the reduction products of nitric acid. The oxygen gas was initially bubbled into the aqueous phase but quickly separated and collected in the headspace of the reactor where it was eventually removed. It was necessary to supply a large amount of oxygen gas to adequately oxygenate the aqueous phase.
Another problem with this process concerned controlling the amount of oxygen gas in the aqueous phase. It was difficult to directly measure the amount of oxygen gas in the aqueous phase. However, it was relatively simple to measure the amount of oxygen gas in the headspace. Consequently, the amount of oxygen gas supplied to the reactor was controlled based on this measurement. Unfortunately, the amount of oxygen gas in the headspace bore a tenuous relationship to the amount of oxygen gas in the aqueous phase. It proved difficult to precisely control the amount of oxygen gas supplied to the aqueous phase.
Other problems associated with this process were manifest when it was attempted to operate it continuously. The reactor was highly pressurized and the pressure fluctuated significantly over time. This made it difficult to introduce feed material into the reactor at a constant rate. The feed material had a tendency to enter in spurts and pauses, which created problems controlling the reaction. Each time a spurt of feed material entered the reactor, a number of parameters would have to be adjusted so that it could remain in the reactor long enough to completely oxidize.
The process was further complicated by variations in the physical characteristics of the feed material, such as particle size, uniformity, moisture content, and the like. These problems were manifest by plugging and clogging at various points up to and including entry into the reactor, unpredictable residence times and reaction rates, process control difficulties, and the like. These problems resulted in oversizing the process equipment and extending the residence times to take into account the inconsistencies between the feed materials.
A number of embodiments of an improved aqueous phase oxidation process are described below. The improved process reduces or eliminates many of the problems and disadvantages associated with conventional aqueous phase oxidation processes.